The present invention relates to a heat storage device for temporarily storing heat.
Basic constructions of conventional heat storage devices are illustrated in FIGS. 11 through 13 in the form of sectional views.
A heat storage device shown in FIG. 11 comprises a heat storage tank 32 charged with a heat storage material 31 such as water, and a heat exchanger 33 disposed in the heat storage tank 32, for executing a heat exchange with the outside. In order to store the heat of, for example, a temperature Tx within this heat storage device, the temperature of the heat storage material 31 is raised to Tx by adding the heat of a temperature of at least Tx to the heat exchanger 33 from the outside. When heat is needed at the outside, the heat of the temperature Tx is extracted from the heat storage material 31 to the outside, using the heat exchanger 33.
A heat storage device as shown in FIG. 12 comprises heat storage tanks 42a and 42b charged with a heat transfer medium 47 such as water which also serves as a heat storage material for storing heat, wherein the heat storage tank 42a has a communicating tube 44a for connecting the heat storage tank 1 with the outside, wherein the heat storage tanks 42a and 42b are connected by a communicating tube 44b, and wherein the heat storage tank 42b has a communicating tube 44c for connecting the heat storage tank 42b with the outside.
In the operation of this heat storage device, in order to store the heat of, for example, a temperature Tx, the heat transfer medium 47 is sucked out from the communicating tube 44c, and the temperature of the heat transfer medium 47 is raised to at least Tx by adding heat to the heat transfer medium 47 at the outside, then the heat transfer medium 47 being injected from the communicating tube 44a to the heat storage tanks 42a. The heat transfer medium 47 removed from the heat storage tank 42a by this injection, arrives at the heat storage tanks 42a passing through the communicating tube 44a. After a while, the heat transfer medium 47 is sucked out from the communicating tube 44c, and executes a heat transport between the heat storage tank 42 and the outside, then returning to the heat storage tank 42a again passing through the communicating tube 44a. Thus, a circulation of the heat transfer medium 47 is accomplished. If heat is needed at the outside, a process in the opposite direction to the injection process of heat is executed. That is, the heat transfer medium 47 is sucked out from the communicating tube 44a, and heat is absorbed and utilized at the outside, and the heat transfer medium 47 of which temperature has decreased as a result of the absorption and utilization of heat, is returned from the communicating tube 44c to the heat storage tank 41b. Due to this returning, the heat transfer medium 47 removed from the heat storage tanks 42b flows into the heat storage tank 42a passing through the communicating tube 44b. Thus, a circulation of the heat transfer medium 47 is accomplished.
An object of suitably dividing the heat storage tank is to suppress the occurrence of a dead water region where flow is apt to stagnate within the heat storage tank, and to allow the circulation of the heat transfer medium 47 in injecting and extracting heat to be uniformly conducted by using the whole of the heat storage tank.
If the heat of a lower temperature than an ambient temperature is stored, the directions of flow of the heat transfer medium 47 in the processes of injection and extraction will be opposite to each other. However, the basic operation is similar to the foregoing.
On the other hand, in a heat storage device shown in FIG. 13, a heat storage material 51 utilizing mainly transition heat is charged into small vessels 56, which are accommodated in the heat storage tank 52, and the heat storage tank 52 is provided with communicating tubes 55a and 55b for making the heat storage tank 52 communicate with the outside. By the heat transfer medium 57 which circulates through these communicating tubes 55a and 55b and the heat storage tank, heat exchange means for executing a heat exchange with the outside is constituted. In this case, the storage of heat is mainly executed by the heat storage material 51 which utilizes a transition heat. Although the heat transfer medium 57 functions as heat transport means, it does not mean that the heat transfer medium 57 does not participate in the storage of heat.
In this heat storage device, in order to store the heat of, for example, a temperature Tx, the heat transfer medium 57 having a temperature of at least Tx is injected from the outside to the heat storage tank 52 through the communicating tube 55a. The heat storage material 51 is heated by the heat which has been released by the heat transfer medium 57 via the walls of the small vessels 56. The heat transfer medium 57 injected returns to the outside passing through the communicating tube 55b, and is again heated to a temperature of at lease Tx. Thus, a similar circulation is repeated. If heat is needed at the outside, a process in the opposite direction to the process of heat injection is executed. That is, the heat transfer medium 57 is injected from the outside to the heat storage tank 52 through the communicating tube 55b. The heat transfer medium 57 injected is heated by the heat released by the heat transfer medium 51, via the walls of the small vessels 56. The heat transfer medium 57 heated returns to the outside passing through the communicating tube 55a, and its temperature becomes lower than Tx as a result of the utilization at the outside, then being injected again. Thus, a similar circulation is repeated. Also in the case where the heat of a lower temperature than the ambient temperature is stored, the basic operation is similar to the foregoing.
In this example of constitution, since the heat transfer medium 57 which is injected and extracted from the outside passes through the gaps between the small vessels, the area of a heat transfer surface to the heat storage material 51 becomes large, as well as the treatment of the heat storage material 51 becomes easy, and hence such a constitution is often used for heat storage devices that use paraffin as a heat storage material 51 which utilizes transition heat.
In the above-described heat storage devices shown in FIGS. 11 through 13, in the state of heat storage, if there are temperature differences between the heat storage materials 31, 51, and the heat transfer medium 47 which also serves as a heat storage material, and the external environment surrounding the heat storage tanks 32, 42, and 52, then heat transfer always takes place between the heat storage devices and the external environment through the wall surfaces of heat storage tanks 32, 42, and 52, respectively.
Neglecting the influence of heat radiation which is usually low, the heat loss Q of a heat storage material or a heat transfer medium which also serves as a heat storage material (hereinafter these are both referred to as a heat storage material) is expressed by the following equation.
Q=kA(Txxe2x80x2xe2x88x92T0)dt [J]xe2x80x83xe2x80x83(1)
Here, k denotes an overall heat transfer coefficient determined by the material, construction, and ambient air speed of a heat storage tank, etc. A denotes a contact area between a heat storage tank and a ambient fluid (such as air). T0 represents an environmental temperature outside the heat storage device, and Txxe2x80x2 represents a temperature at the surface where the heat storage material contacts the heat storage tank (the surface of heat storage material), t representing an elapsed time. Since k and A can usually be regarded as constants irrespective of time, the equation (1) may be expressed approximately as follows:
Q=kA(Txxe2x80x2xe2x88x92T0)dt [J]xe2x80x83xe2x80x83(2)
Some heat storage methods mainly utilize a sensible heat of the heat storage material 31 or the heat transfer medium 47 as in the cases shown in FIGS. 11 and 12, and other heat storage methods mainly utilize a transition heat of the heat storage material 51 as in the case shown in FIG. 13. Whichever heat of a sensible heat or a transition heat may be utilized, in order to store the heat of, for example, a temperature of Tx, it is necessary for the surface temperature Txxe2x80x2 of the heat storage material to be maintained at a temperature of at least Tx. However, the greater is the temperature difference between the surface temperature Txxe2x80x2 of the heat storage material and the ambient temperature T0 in the outside, and also the longer is the storage time, the larger the heat loss represented in the equation (2) becomes, which results in a marked reduction in heat storage efficiency.
In order to reduce the heat loss from the heat storage tank in the equation (2), therefore, it is necessary to reduce the overall heat transfer coefficient k and/or the surface area A and shorten the time t, or to reduce the difference between the surface temperature of the heat storage material Txxe2x80x2 and the environmental temperature T0.
As a method for reducing the overall heat transfer coefficient k, an attempt to install a heat insulator around the periphery of the heat storage tank has been made. As a method for reducing the surface area, there has been made an attempt to make the heat storage tank have a shape having a small surface area per a unit volume, such as a cube or a sphere.
As a method for shorten the time t, there has been made an attempt to optimize the system control over heat application system after the re-extraction of heat. However, since the time term and temperature term in the equation (2) are associated with the original purpose of the heat storage, they can not be widely changed by nature.
On the other hand, in the examples shown in FIGS. 11 and 12, since the temperature of the heat storage material gradually approaches that of the external environment from the portions closer to the wall surfaces of the heat storage tanks 32 and 42, a temperature difference is generated within the heat storage material. In general, since a matter expands or contracts as it changes in temperature, once a temperature difference is generated in the heat storage material, a density difference occurs therein, and a gravity difference is caused. As a consequence, a movement called a natural convection is induced within the heat storage material.
Therefore, a natural convection takes place from the vicinity of the wall surfaces of the heat storage tank 32 and 42 that have the highest temperature difference between the same and the external environment, that is, from the outer portions in the heat storage material, and it spreads throughout the heat storage material. Consequently, heat storage material having a temperature closer to the temperature of external environment rather than to the heat stored flows from the surface side into the inner portion of the heat storage material, and simultaneously, the heat stored in the surface of the heat storage material transfers, whereby heat transfer always takes place between the heat storage material and the external environment through the wall surface of the heat storage tank, and hence causes a large heat loss within the heat storage material.
In the above-described heat storage device shown FIG. 13, in the conservation process of heat, the natural convection of the heat storage material 51 within the heat storage tank 52 is suppressed by small vessels 56, but a natural convection is generated within the heat transfer medium 57 stagnating in the heat storage tank 52. Accordingly, with regard to the heat transfer medium 57, the same as the case of the heat storage materials 31 and 47 of the heat storage devices shown in FIGS. 11 and 12 holds true.
In this way, since the temperature of the heat storage material changes to the temperature close to that of the external environment from the heat storage material in the portion closer to the wall surface of heat storage tanks, a temperature difference is generated between the outer and inner portions within the heat storage material (in the case of spherical heat storage tank, in the radial direction) in the process of heat transfer. Consequently, since a natural convection is induced within the heat storage material, heat transfer always takes place between the heat storage material and the external environment through the wall surface of heat storage tank, incurring a heat loss of the heat storage material.
In the above-described conventional heat storage devices shown in FIGS. 11 through 13, since the heat storage materials tend to easily flow in any case, a natural convection throughout the heat storage material is prone to occur not only in the process of conservation of heat, but also particularly in the processes of the injection and extraction of heat due to a temperature difference as a result of heat exchange. This incurs an increase in the overall heat transfer coefficient k in the equation (2), and an increase in the heat loss from the heat storage material to the outside, which constitutes one of the factors reducing a heat recovery rate.
The present invention has been achieved to overcome the heat loss from a heat storage material as described above and aims to provide a heat storage device capable of reducing the influence of external environment on the inside portion of a heat storage tank and suppressing the heat loss toward the outside, by reducing the temperature difference between the surface of the heat storage material and the external environment, and at the same time, by suppressing the heat transfer between the surface of the heat storage material and the inside portion of the heat storage material which executes injection and extraction between the same and the external environment.
A first heat storage device for achieve the above-mentioned object comprises: a heat storage tank charged with a heat storage material for storing the heat provided from the outside; and heat exchange means which executes the injection and extraction of heat between the inside of a heat storage tank and the outside by the heat exchange between the heat storage material and the heat transfer medium, wherein the heat exchange means is disposed so as to execute a heat exchange between the central portion of the heat storage tank and the outside, or so that the central portion and the outer portion in the heat storage tank being caused to perform a heat exchange with the outside sequentially or individually, and wherein suppressing means are disposed in the outer portion in the heat storage tank, for suppressing the natural convection of the heat storage material.
A second heat storage device for achieve the above-mentioned object comprises: a heat storage tank charged with a heat transfer medium which also serves as a heat storage material for storing heat supplied from the outside; and heat transport means which executes an injection and extraction of heat between the inside of the heat storage tank and the outside by the inflow and outflow of the heat transfer medium, wherein the heat transport means is disposed so as to execute a heat transport between the central portion of the heat storage tank and the outside, and wherein suppressing means are disposed in the outer portion in the heat storage tank, for suppressing the natural convection of the heat storage material.
In the above-described heat storage devices, suppressing means may be constituted by: dispersing a liquid-absorbent material into the heat storage material or heat transfer medium in the outer portion in the heat storage tank; providing the heat storage material or heat transfer medium in the outer portion in the heat storage tank with a property of increasing viscosity by the application of voltage, and providing the heat storage device with means for applying power between a pair of electrodes which are disposed on opposite sides of the outer portion in the heat storage tank so as to sandwich the outer portion; or providing the heat storage material or heat transfer medium in the outer portion in the heat storage tank with a property of increasing viscosity by the application of magnetic force, and providing the heat storage device with a magnet for exerting a magnetic force on the outer portion in the heat storage tank; and further providing the outer portion in the heat storage tank with a barrier for hindering the natural convection of the heat storage material or heat transfer medium.
Also, in the above-described heat storage devices, there can be provided means for promoting the heat transfer between the central portion and the outer portion in the heat storage tank.
In the heat storage device having the above-described constitution, the heat of a temperature more than necessary at the heat extraction is injected to the heat storage tank by heat exchange means which execute a heat exchange with the central portion in the heat storage tank or by heat transport means which transport heat to the central portion, and then the heat is transferred to, stored in, and conserved in the heat storage material or heat transfer medium which is accommodated mainly in the outer portion in the heat storage tank. As necessary, the heat stored is extracted from the central portion of heat storage tank.
When injecting or extracting heat, since a heat exchange is executed mainly in the central portion in heat storage tank where convection easily occurs, a large heat transfer due to natural convection heat transfer joins in the heat transfer due to a heat conduction, and thereby an efficient heat exchange with the outside is achieved.
When conserving heat, since a heat transfer is usually generated between the outer portion in the heat storage tank and the external environment, a temperature difference due to heat transfer is generated in the heat storage tank, and consequently, a natural convection tries to arise from the heat storage material or heat transfer medium located closer to the heat storage tank. However, in the above-described heat storage devices, the suppressing means for suppressing a natural convection, disposed in the outer portion in the heat storage tank, make it difficult for the outer portion to perform a large heat transfer due to natural convection.
As described above, in the present invention, the natural convection is suppressed in the outer portion in the heat storage tank, and the natural convection generated on the surface of the heat storage material in the outer portion in the heat storage tank is hindered from extending throughout the heat storage material in the outer portion in the heat storage tank, and consequently, the heat transfer and overall heat transfer coefficient (k) due to the convection between the outer portion in the heat storage tank and the external environment is reduced, and at the same time, the heat within the heat storage material in the outer portion in the heat storage tank is hindered from transferring to the surface of the heat storage material. This creates a great temperature gradient within the heat storage material in the outer portion, and gradually reduces the temperature gradient between the surface of heat storage material and the external environment.
It is therefore possible to reduce the heat loss from the inside of the heat storage tank to the external environment while maintaining the temperature of the inside of the heat storage tank substantially constant.
In the present invention, the suppressing means against natural convection can be constituted by dispersing a liquid-absorbent material into the heat storage material or heat transfer medium in the outer portion in the heat storage tank. In this case, the above-mentioned liquid-absorbent material stagnates in the form of three-dimensional meshes in the outer portion in the heat storage tank, by adsorbing a part of the heat storage material or heat transfer medium and expanding. That is, since the heat storage material or the like comes to fill gaps within the liquid-absorbent material so as to be difficult to move, the natural convection of the heat storage material in the outer portion is suppressed, which enables the reduction in the heat loss from the inside of the heat storage tank to the outside.
As the above-mentioned suppressing means, when providing the heat storage material or heat transfer medium in the outer portion in the heat storage tank with a property of increasing viscosity by the application of voltage, and means for applying power between the pair of electrodes which are disposed on opposite sides of the outer portion in the heat storage tank so as to sandwich the outer portion, an electric field is generated in the outer portion by the application of voltage, and thereby the viscosity of the heat storage material is increased. This hinders a free movement of the heat storage material, and allows the suppression of the natural convection of the outer portion. Also, as suppressing means, when providing the heat storage material or heat transfer medium in the outer portion in the heat storage tank with a property of increasing viscosity by the application of a magnetic force, and providing a magnet for exerting a magnetic power on the outer portion in the heat storage tank, the viscosity of the heat storage material or the heat transfer medium is increased by the action of the magnetic field of the magnet. This hinders a free movement of the heat storage material, and allows the suppression of natural convection of the outer portion.
Further, the suppressing means can be constituted as a barrier disposed in the outer portion in the heat storage tank, for hindering the natural convection of the heat storage material or the heat transfer medium. In this case, since the movement of the heat storage material or heat transfer medium can physically be hindered by the barrier, appropriately disposing the barrier permits the moving speed of the heat storage material to be significantly lower than the case without hindrance, which results in the suppression of natural convection.
In addition, in the present invention, means for promoting heat transfer is provided between the central portion and the outer portion in the heat storage tank. When, for example, a rapid injection to and an extraction from the heat storage tank is required, it is possible to speedily execute a heat transfer between the central portion and the outer portion where the heat transfer from the central portion is difficult due to the suppression means against natural convection.
The heat exchange means in the present invention can be disposed so that the central portion and the outer portion in the heat storage tank are caused to sequentially perform a heat exchange with the outside. By this constitution, when injecting heat, by utilizing the residual heat remaining after the heating or cooling of the central portion in the heat storage tank, it is possible to additionally heat or cool the outer portion. On the contrary, when extracting heat, after the preheating or precooling of the heat exchanger in the outer portion in the heat storage tank, it is possible to additionally heat or cool the heat exchanger in the outer portion. This allows the exchange of heat between the heat storage device and the external environment to be executed rapidly and efficiently.
Moreover, the heat exchange means can be disposed so that the central portion and the outer portion in the heat storage tank are caused to individually perform a heat exchange with the outside. By this constitution, when injecting heat, a heat exchange can be executed in the central portion or the outer portion whichever is more suitable, in accordance with the temperature of the heat injected from the outside to the heat storage tank. On the contrary, when extracting heat, a heat exchange may be executed in the central portion or the outer portion whichever is more suitable, in accordance with the temperature of the heat required by the outside. Consequently, the exchange of heat between the heat storage device and the external environment can be executed rapidly and efficiently.
In accordance with the above-described heat storage devices associated with present invention, since the ease of convection of the heat storage material or heat transfer medium within the heat storage tank varies depending on position, the injection operation from the outside to the heat storage material and the a extraction operations from the heat storage material to the outside are executed toward the central portion where a natural convection is easy to occur, while the storage of heat is executed in the central portion where a natural convection is difficult to occur. Therefore, in the processes of the injection and extraction of heat as well as in the process of conservation, the natural convection occurring within the inside of the heat storage material due to the temperature difference from the outside takes place in the central portion in the heat storage tank and hardly takes place in the outer portion, with the result that the overall heat transfer coefficient k becomes less than conventional cases. This enables the provision of a heat storage device having a low heat loss toward the outside.
Also, providing the central portion and the outer portion with heat transfer elements permits the heat transfer between the central portion and the outer portion where heat transfer does not much occur basically to be temporarily promoted, whereby it is possible to provide a heat storage device having a low heat loss as described above while executing an efficient operating of the heat storage tank in accordance with the supply and demand of heat.
Further, disposing heat exchangers so as to make a circuit through the central portion and the outer portion in the heat storage tank, permits the heat exchanger to execute a direct heat exchange with the outer portion where heat transfer does not much occur basically, and to execute an injection or an extraction of heat between the central portion and the outer portion sequentially, whereby it is possible to provide a heat storage device having a low heat loss as described above while executing the exchange of heat between the heat storage device and the outside rapidly and on a large scale. Also, individually disposing heat exchangers in the central portion and the outer portion enables a selective use of either the central portion or the outer portion each having a different temperature, or both of them, whereby it becomes possible to provide a heat storage device having a low heat loss while efficiently executing an heat change between the heat storage device and the outside in accordance with the supply and demand of heat.